This invention relates to a servo circuit for normalizing error signals, such as one for controlling an optical spot in an optical disks such as found in, for example, video disks, compact disks and other such devices.
FIG. 1 is a block diagram of a conventional servo circuit which is used as an automatic tracking servo system in an optical disk device. In FIG. 1, reference numeral 16 designates a laser beam source for emitting, for instance, a beam generated by a semiconductor laser. A reflecting plate 17 deflects the laser beam emitted by the laser beam source 16 through 90.degree. (i.e., to reflect the laser beam in the direction of the arrows). The reflecting plate 17 can be rotated about a rotating shaft 18. Photo detectors 19 and 20 receive a laser beam reflected by the reflecting plate 17 and output respective photoelectric curents corresponding to quantities of light received. The photodetectors 19 and 20 are disposed on either side of the central axis of the laser beam, which axis is indicated by a dot-dash line in FIG. 1. Current to voltage converters 21 and 22 respectively convert the photoelectric currents output by the photodetectors 19 and 20 into voltage signals. The photodetectors 19 and 20 together with the current to voltage converters 21 and 22 comprise a detector means.
A differential amplifier unit or signal processing means 23 respectively receives at input terminals 10 and 11 the voltage signals provided by the converters 21 and 22. The voltage signals are applied to a differential amplifier 12 and an adder 13. The output of the amplifier 12 and the output of the adder 13 are applied to an analog division unit 14, which provides, at an output terminal 15, a servo signal. In response to the servo signal from the analog division unit 14, a control unit 24 controls a motor 25 so as to control the rotational angle of the reflecting plate 17 and to keep the central axis of the laser beam midway between photodetectors 19 and 20. The rotating plate 17, rotating shaft 18, motor 25 and control unit 24 comprise a position actuator means.
The operation of the above servo circuit is described below. The laser beam from the laser beam source 16 is reflected by the reflecting plate 17 and received by the two photodetectors 19 and 20. The photodetectors 19 and 20 output respective photo electric currents varying in response to the quantity of light received. The photoelectric currents are converted into voltage signals by the current to voltage converters 21 and 22, respectively. These voltage signals are then applied to the differential amplifier unit 23. In the differential amplifier unit 23, the differential amplifier 12 provides a difference output, which indicates for instance, a shift of the optical axis from the disk track. The adder 13 provides a total detection output, i.e., a sum output. In the analog division unit 14, the sum output of the adder 13 divides the difference output of the differential amplifier 12, to provide a servo signal with a normalized output level. The output of the analog division unit 14 is applied to the control unit 24 to drive the motor 25 so that the optical axis is kept on the division line between the photodetectors 19 and 20.
The servo circuit described above is a tracking servo circuit. A focusing servo circuit has substantially the same circuit arrangement as the tracking servo circuit, and is used for moving an objective lens vertically to focus it on an object.
The above servo circuit requires an expensive analog division unit for normalization, i.e., division. Furthermore, an analog signal is normalized, and therefore drifts or changes greatly with time due to, for example, time and temperature induced changes in the analog circuit components. Thus, the servo circuit has rather low reliability.
In addition, because the above servo circuit comprises analog circuits, offset quantities (i.e., the difference between the position indicated by the photodetector and the actual position of the optical spot) must be eliminated. Such errors arise from optical system assembly errors or the like. The elimination of such errors requires adjustments to be performed for every servo circuit by applying a voltage through a variable resistor or the like to the circuit. Thus, the abovedescribed servo circuit has poor accuracy and instability, and is affected by temperature changes and time.